The present invention concerns steel powder compositions as well as the compacted and sintered bodies obtained thereof. Specifically the invention concerns stainless steel powder compositions for warm compaction.
Since the start of the industrial use of powder metallurgical processes i.e. the pressing and sintering of metal powders, great efforts have been made in order to enhance the mechanical properties of P/M-components and to improve the tolerances of the finished parts in order to expand the market and achieve the lowest total cost.
Recently much attention has been paid to warm compaction as a promising way of improving the properties of P/M components. The warm compaction process gives the opportunity to increase the density level, i.e. decrease the porosity level in finished parts. The warm compaction process is applicable to most powder/material systems. Normally the warm compaction process leads to higher strength and better dimensional tolerances. A possibility of green machining, i.e. machining in the xe2x80x9cas-pressedxe2x80x9d state, is also obtained by this process.
Warm compaction is considered to be defined as compaction of a particulate material mostly consisting of metal powder above approximately 100xc2x0 C. up to approximately 150xc2x0 C. according to the currently available powder technologies such as Densmix(trademark), Ancorbond(trademark) or Flow-Met(trademark).
A detailed description of the warm compaction process is described in e.g. a paper presented at PM TEC 96 World Congress, Washington, June 1996, which is hereby incorporated by reference. Specific types of lubricants used for warm compaction of iron powders are disclosed in e.g. the U.S. Pat. Nos. 5,154,881 (Rutz) and 5,744,433 (Storstrxc3x6m).
Until recently it has been observed that the general advantages with warm compaction have been insignificant as only minor differences in e.g. density and green strength have been demonstrated in the case of stainless steel powders. Major problems encountered when warm compacting stainless steel powders are the high ejection forces and the high internal friction during compaction.
However, as disclosed in the U.S. Pat. No. 6,365,095 (Bergkvist), it was recently found that stainless steel powders may be subjected to warm compaction with good results provided that the stainless steel powder is distinguished by very low oxygen, carbon and silicon levels. The widely used standard qualities having higher levels of these elements could however not be successfully warm compacted i.e. the properties of the warm compacts were not significantly better than the green density of a corresponding body compacted at ambient temperature.
It has now unexpectedly been found that also standard stainless steel powders can be compacted at elevated temperatures with good results. In comparison with the stainless steel powders disclosed in the above US patent the standard stainless powders are generally characterised in a higher amount of oxygen, carbon and silicon. These powders are also easier to produce and accordingly cheaper. According to the present invention it has thus, contrary to the teaching in the SE publication, been found that these standard powders can be compacted to high green densities without the use of excessively high compaction pressures. The high green density is valuable when the product is subsequently sintered as it is not necessary to use high sintering temperatures and accompanying high energy consumption in order to get a high sintered density which is normally necessary in order to get good mechanical properties. Additionally high sintering temperatures induce strains in the material which in turn gives poor dimensional stability.
In brief the process of preparing high density, warm compacted bodies of a water atomised standard stainless steel powder according to the present invention is based on the discovery that specific amounts of lubricants have to be used in the stainless steel powder composition which is subjected to the compaction at elevated temperature. Minor amounts of selected additives included in the composition contribute to the unexpected finding that standard stainless steels can be successfully compacted.
Preferably the powders subjected to warm compaction are pre-alloyed, water atomised powders which include, by percent of weight, 10-30% of chromium. The stainless steel powder may also include other elements such as, molybdenum, nickel, manganese, niobium, titanium, vanadium. The amounts of these elements may be 0-5% of molybdenum, 0-22% of nickel, 0-1.5% of manganese, 0-2% of niobium, 0-2% of titanium, 0-2% of vanadium, and at most 1% of inevitable impurities and most preferably 10-20% of chromium, 0-3% of molybdenum, 0.1-0.4% of manganese, 0-0.5% of niobium, 0-0.5% of titanium, 0-0.5% of vanadium and essentially no nickel or alternatively 5-15% of nickel, the balance being iron and unavoidable impurities (normally less than 1% by weight). Examples of stainless steel powders which are suitably used according to the present invention are 316 LHC, 316 LHD, 409 Nb, 410 LHC, 434 LHC. The standard steel powders used according to the present invention generally include more than 0.5% by weight of Si and normally the Si content is 0.7-1.0% by weight. This feature distinguishes standard stainless powders from the stainless powders used for the warm compaction according to the U.S. Pat. No. 6,365,095 (Bergkvist) mentioned above.
The amount of lubricant in the composition to be compacted is an important factor for the possibility to get a satisfactory result. It has thus been found that the total amount of lubricant should be above 0.8% by weight, preferably at least 1.0% by weight and most preferably at least 1.2% by weight of the total powder composition. As increasing amounts of lubricant decrease the final green density due to the fact that the lubricants normally have much lower density than the metal powder, lubricant amounts above 2.0% by weight are less important. In practice it is believed that the upper limit should be less than 1.8% by weight. A minor amount, such as at least 0.05 and at most 0.4% by weight of the lubricant should preferably be a compound having high oxygen affinity.
The lubricant may be of any type as long as it is compatible with the warm compaction process. Examples of such lubricants are disclosed in e.g. the U.S. Pat Nos. 5,154,881 (Rutz) and 5,744,433 (Storstrxc3x6m), which are referred to above and which are hereby incorporated by reference. Preliminary results have also shown that lubricants conventionally used for cold compaction, such as EBS, may be used for warm compaction of the standard steel powders according to the present invention although the flow properties of such powder compositions are inferior.
So far however the most promising results have been obtained by using a type of lubricants disclosed in the copending patent application SE02/00762 PCT. These type of lubricants include an amide component which can be represented by the following formula
Dxe2x80x94Cmaxe2x80x94Bxe2x80x94Axe2x80x94Bxe2x80x94Cmbxe2x80x94D
wherein
D is xe2x80x94H, COR, CNHR, wherein R is a straight or branched aliphatic or aromatic group including 2-21 C atoms
C is the group xe2x80x94NH (CH)nCOxe2x80x94
B is amino or carbonyl
A is alkylen having 4-16 C atoms optionally including up to 4 O atoms
ma and mb which may be the same of different is an integer 1-10
n is an integer 5-11.
Examples of preferred such amides are:
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]2xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]2xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]2xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]3xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]3xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]3xe2x80x94OCCH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]3xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]4xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]4xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]4xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]4xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]5xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]5xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]5xe2x80x94OC(CH2)16CH3 
As previously mentioned the lubricant should preferably also include a compound having high affinity for oxygen. Examples of such high affinity compounds are alkali metal stearates. Other examples are stearates of alkaline earth metals. The presently most preferred compound being lithium stearate.
According to a preferred embodiment of the invention minor amounts of selected additives may be included in the composition before the powder composition is subjected to warm compaction. These additives include fatty acids and flow enhancing agents.
The fatty acid may be selected from the group consisting of stearic acid and oleic acid. The amounts of the fatty acid in the composition according to the invention may vary between 0.005 and 0.5, preferably between 0.010 and 0.16 and most preferably between 0.015 and 0.10% of the lubricant composition.
The flow agent may be a material of the type described in the U.S. Pat. No. 5,782,954 (Luk). This material is comprised of nanoparticles of various metals and their oxides such as silicon oxide. Typically, the metal and metal oxide powders have average particle sizes below about 500 nanometers. The silicon oxide flow agents are preferably blended with the iron-based powders in an amount of from about 0.005 to about 2 percent by weight of the resultant powder composition. The preferred silicon oxide flow agents are powders or particles of silicon dioxide having an average particle size below about 40 nanometers. An example of a suitable flow agent is Aerosil.
The stainless steel powder including the lubricant and optional additives is subsequently compacted at an elevated temperature. The warm compaction may be performed with a preheated powder, a preheated die or both. The powder could e.g. be preheated to a temperature between 100xc2x0 C. and 200xc2x0 C. and the compaction could be performed at a temperature of about 100xc2x0 C. and 150xc2x0 C. The compaction is performed in standard compaction equipment with compaction pressures preferably between about 500 and 800 MPa.
The obtained green bodies are then sintered in the same way as the standard materials, i.e. at temperatures between 1100xc2x0 C. and 14000xc2x0 C., the most pronounced advantages being obtained when the sintering is performed between 1250 and 1325xc2x0 C. A lower sintering temperature may be used in order to reach a given sintered density by using warm compaction instead of compaction at ambient temperature. Furthermore the sintering is preferably carried out in standard non oxidative atmosphere for periods between 15 and 90, preferably between 20 and 60 minutes. The high densities according to the invention are obtained without the need of recompacting, resintering and/or sintering in inert atmosphere or vacuum.